This application is based on patent application Hei.11-23095 filed in Japan, the contents of which are hereby incorporated by references.
1. Field of the Invention
This invention relates to an optical system used in an optical measuring apparatus for guiding rays emitted from an object to be measured and having exit angles equal to or smaller than a predetermined angle to a photosensing plane of a photosensor in the optical measuring apparatus.
2. Description of the Related Art
An optical measuring apparatus has conventionally been used in a manufacturing line of a liquid crystal display panel (hereinafter abbreviated as LCD panel) for measuring optical characteristics such as chromaticity and luminance. Adjustment of white balance, quality control and shipping inspection of the LCD panel are executed by using the result of the measurement of the optical characteristics.
A conventional optical measuring apparatus comprises a photosensor for receiving rays emitted from a region of an LCD panel which is to be measured (hereinafter abbreviated as measurement region) and an optical system for guiding exit rays from the measurement region to the photosensor. A main control circuit of the apparatus calculates xyY (chromaticity coordinates and luminance) and Txcex94 uvY (correlated color temperature, color difference and luminance), which are established by Commission Internationale de Ixe2x80x2 Eclairage (CIE), by basing on output of the photosensor of the probe. The calculated values are displayed on a display panel of the apparatus.
On the other hand, it is conventionally known that the chromaticity and the luminance of the image displayed on the LCD panel varies when an observer moves from the front center of the LCD panel to the side thereof. This phenomenon is caused by light distribution of the LCD panel, that is, the larger the exit angle of the rays emitted from the LCD panel becomes, the smaller the intensity of the light becomes.
A light distribution of a typical LCD panel on the market is shown in FIG. 19. The light distribution of the LCD panel is substantially symmetrical with respect to the normal xe2x80x9cNxe2x80x9d at the center of the LCD panel 12 in both of the vertical direction XX and the horizontal direction YY, so that an image can be displayed preferably when it is observed from the front center thereof. However, when the LCD panel is observed from the side at an observing angle larger than a predetermined angle "Egr" against the normal xe2x80x9cNxe2x80x9d, the chromaticity and the luminance of the image are largely varied. It is generally said that the angle of view of the LCD panel is narrow.
In the conventional optical measuring apparatus, the photosensor is positioned distant from the LCD panel, so that the photosensor receives not only paraxial rays but also the rays having exit angles larger than the predetermined angle xcex1 (in the following description, the angle xcex1 is called xe2x80x9cmaximum exit anglexe2x80x9d). Thus, the accuracy and the repeatability of the measurement of the optical characteristics of the LCD panel by the conventional optical measuring apparatus are not so high.
Such the problem is commonly exists in the optical systems used for measuring light sources having light distribution in which intensity or luminance of light is largely varied when exit angles of rays becomes larger than a predetermined angle.
A purpose of this invention is to provide an optical system suitable for optical measuring apparatus for guiding rays emitted from an object to be measured and having exit angles equal to or smaller than a predetermined maximum exit angle to a photosensor without any affect due to light distribution of the object.
An optical system in accordance with an aspect of the present invention comprises a lens with positive power for guiding rays emitted from an object to be measured and having exit angles equal to or smaller than a predetermined angle to a photosensing plane of a photosensor. The lens is disposed between the object and the photosensor in a manner so that a ray emitted in the normal direction from an end of the object crosses another ray emitted with a maximum exit angle from another end the object at an end of the photosensing plane of the photosensor.
By such a configuration, only the rays emitted from the object and having the exit angle equal to or smaller than the maximum exit angle can reach to the photosensing plane of the photosensor.
Furthermore, it is preferable that the lens satisfies the following two equations.
f=|2xc2x7h/tan xcex1|
L=(Hxe2x88x92h)xc2x7f/H
Hereupon, the symbol xe2x80x9cfxe2x80x9d designates a focal length of the lens; the symbol xe2x80x9chxe2x80x9d designates the maximum height of the photosensitive plane of the photosensor from an optical axis of the lens on the photosensing plane; the symbol xe2x80x9cLxe2x80x9d designates a distance from a principal point of the lens in the image side to the photosensing plane; and the symbol xe2x80x9cHxe2x80x9d designates the maximum height of the object from the optical axis of the lens.